Cellulose, which is obtained from various types of wood, has many uses. One use, as is well known in the art, is to convert the cellulose in the presence of acetic anhydride, acetic acid, and sulfuric acid, into the triacetate, followed by partial hydrolysis of the triacetate to remove some of the acetate groups and to degrade the chains into smaller fragments of about 200-300 units each, in order to yield the commercially important product, cellulose acetate.
Cellulose acetate is not as flammable as cellulose nitrate, and hence, has replaced the nitrate in many of its applications. For instance, cellulose acetate is used in the manufacture of photographic film, as well as in the manufacture of filaments used in the textile industry.
Commercially available dissolving pulps have a high content of about 90-98% cellulose (also known as the content of glucan, a polymer containing repeating units of glucose), a low content of hemicelluloses (e.g., mannan or xylan), very little residual lignin, a low extractive and mineral content, a high brightness, and preferably, a uniform molecular weight distribution. Thus, due to the high cellulose content, the major use of dissolving pulps is in the manufacture of cellulose acetate although there are other uses.
Today, most dissolving pulps are derived from wood using the acid sulfite process or the prehydrolysis kraft process. The objective in using these processes to make dissolving pulps from cellulose is to remove or to degrade both lignin and hemicelluloses to enhance their removal by alkali extraction.
More particularly, the acid sulfite process uses a higher temperature and higher acidity as compared to cooking conditions for producing paper products. On the other hand, the prehydrolysis kraft process applies an acidic pretreatment before the alkaline pulping stage in order to degrade the hemicelluloses, consequently allowing for their easier removal during the cook. Cold alkali extraction in a subsequent bleaching sequence can remove residual hemicelluloses to obtain pulp grades for high quality end uses.
In softwoods, cold alkali extraction of prehydrolysis kraft pulps can achieve pulps with mannan levels of approximately 1%, whereas cold alkali extraction of hardwood pulps can reach mannan and xylan levels of approximately 0.5% and 1%, respectively. Low levels of these 2 hemicelluloses are desirable because as the levels both become higher, for instance if the mannan is above about 1.6% or if the xylan is above about 2.7%, a milky or cloudy appearance occurs in the cellulose acetate. For uses of dissolving pulps other than in the manufacture of cellulose acetate, the cloudiness is of less importance and may not be a factor at all, so that a mannan content of 1.5% and a xylan content of 2.6% in dissolving pulps may be acceptable.
In contrast to the prehydrolysis kraft process for making dissolving pulps, the conventional kraft process stabilizes residual hemicelluloses against further alkali attack. This stabilization prevents the production of acceptable quality dissolving pulps through treatment in the bleach plant.
Several decades ago, cold alkali extraction of softwood kraft pulps had been used to produce dissolving grade pulps. Nitration grade pulps (i.e., those employed in the production of the above-noted more flammable cellulose nitrate) were produced in Sweden and Australia as early as the 1930's, as well as having been produced in the United States during World War II. See, for instance, "Chemical Cellulose from Radiata Pine Kraft Pulp", Wallis et al., Vol. 43, No. 5, Appita, p. 355 (1990).
As also reported by Wallis et al. in "Chemical Cellulose from Radiata Pine Kraft Pulp", bleached radiata pine kraft pulps and radiata pine bisulfite pulps have been converted to nitration grade dissolving pulps. Such conversions have also been reported by Evans et al., in an article entitled "CEH Bleached Radiata Pine Bisulfite Pulp as a Source of Chemical Cellulose", published in Vol. 43, No. 2, Appita, pp. 130-136 (1990). More particularly, both groups reported that cold alkali extraction of the bleached pulps with 8-10% NaOH for 1 hour at 20.degree. C. gave products with satisfactory chemical properties for nitration grade cellulose. However, this approach of a cold alkali extraction is not applicable to making dissolving pulp suitable for the manufacture of acetylation grade cellulose due to the presence of a resistant xylan fraction remaining after the cold alkali extraction, as also reported by Wallis et al. in "Chemical Cellulose from Radiata Pine Kraft Pulp". As noted above, a high level of the hemicellulose, xylan, is undesirable due to its causing a cloudy appearance in the cellulose acetate.
During the last decade or so, various studies have focused on removing or modifying hemicellulose in bleached chemical fiber with xylanases. For instance, as reported by Paice et al. in "Removing Hemicellulose from Pulp by Specific Enzyme Hydrolysis", Vol. 4, No. 2, J. of Wood Chem. and Tech., p. 187 (1984), xylan was removed from bleached hardwood sulfite pulp with a xylanase preparation from Schizophyllum commune. The xylanase was not very effective in removing xylan from the bleached fiber as only a small decrease in pentosan content was reported.
Similarly, Senior et al., as reported in Vol. 10, No. 12, Biotechnology Letters, pp. 907-912 (1988), performed a study using bleached hardwood kraft pulp and a xylanase from Trichoderma harzianum. Although they observed a 50% decrease in the xylan content of the pulp, the final product still contained a high xylan level that was unacceptable. Apparently, the bleaching process either removed the more accessible hemicelluloses or the modification of the hemicelluloses during pulping produced polysaccharides not recognized by the enzyme.
Additionally, as reported by Mora et al. in "Action of Xylanases on Chemical Pulp Fibers: Part I: Investigations on Cell Wall Modifications", Vol. 6, No. 2, J.of Wood Chem. and Tech., pp. 147-165 (1986) and by Noe et al. in "Action of Xylanases on Chemical Pulp Fibers: Part II: Enzymatic Beating", Vol. 6, No. 2, J. of Wood Chem. and Tech., p. 167-184 (1986), the modification of chemical fibers with xylanases was studied. Mora et al. suggested that xylans are more widely hydrolyzed in the fiber cell wall than is suggested by the hydrolysis rate based upon the release of soluble sugars. More particularly, they concluded that the sugar products do not account for xylan hydrolysis in the fiber cell wall where the polysaccharide is physically retained through hydrogen bonding.
The Mora et al. study was extended by Noe et al. in order to determine whether xylan modifications in the fiber cell wall by xylanases affected paper properties. They reported not only that both bleached kraft birchwood pulp and bleached sulfite spruce pulp demonstrated enhanced beatability and fiber flexibility with a reduction of fiber intrinsic strength due to hydrolysis of xylan, but also that these modifications occurred with only a slight yield loss.
Similar findings have been reported by Roberts et al. in "Modifications of Paper Properties by Treatment of Pulp with Saccharomonospora viridis Xylanase", Vol. 12, No. 3, Enzyme Microb. Technol., pp. 210-213 (March, 1990). Roberts et al. found that xylanase treatment of bleached kraft birchwood pulp resulted in a decrease (25-30%) in burst strength and breaking length with a slight loss (4%) in zero span breaking length. However, the xylan removal was only a low level of 20%, which was attributed by Roberts et al. to poor enzyme accessibility to xylan located in fiber pores.
In contrast, the xylanase method of the present invention overcomes the prior art problems described above in that the resultant dissolving pulp has both a desirably low level of xylan and a desirably low level of mannan.